Semiconductor lasers are in widespread use for technologies such as a light source for communication systems, compact disc players, and so forth. Recently, a new type of laser device the VCSEL has been introduced. The advantages of the VCSEL are that it is smaller, has potentially higher performance, and is more manufacturable. These benefits are due in part from advances in epitaxial deposition techniques, such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Other benefits of VCSELS include geometry, amenability to one-and-two dimensional arrays, wafer-level qualifications and desirable beam characteristics.
VCSELs are designed to have an active region with one or more quantum well layers. The sides of the active region are mirror stacks which are typically formed by interleaved semiconductor layers having properties that require each layer to be a quarter of a wavelength thick. The mirrors in the mirror stacks act as the mirrors of the laser cavity. Opposite conductivity type regions are positioned on opposite sides of the active region. The laser either can be turned “on” and “off” by changing the current through the active region.
VCSELs are useful for applications using fast, medium distance or multi-channel data link applications. VCSELs are also useful for numerous optical and/or imaging applications. The geometry of the VCSELs enable the VCSELs to serve as low cost high performance transmitters with flexible and desirable characteristics.
A problem with multimode lasers is the mode competition induced by spatial and spectral power instability. In many cases, the modal instability is exhibited after an initial burn-in or after a few months of regular operations. Thus, this poses a severe problem for commercial use of VCSELs. The instability leads to different modes seeing different gains, which in turn results in different speeds for the respective modes. Various methods to address the problem have been devised. These methods include (a) increasing the number of modes, (b) performing mode discrimination, and (c) using “peaking circuits”.
The use of a very large number of modes can cause some problems associated with coupling the light from the VCSEL to the multi-mode fiber, and hence, can complicate the design of the coupling optics. The complication stems from the different modes having different divergence angles. As a result, collecting light from all the modes requires a complicated lens design.
The mode discrimination approach also has drawbacks. The mode discrimination approach requires very tight tolerances during manufacturing and packaging. In addition, by suppressing some modes and enhancing other modes in this approach, the overall coupled power to the fiber is significantly reduced. In addition, this approach is very dependent on the exact VCSEL design and structure that is chosen. Hence, this approach cannot easily be applied to a wide variety of VCSEL designs.
Electronic peaking has disadvantages as well. This approach requires additional components on the circuit board. Also, electronic peaking cannot work universally for all VCSELs because dynamic characteristics generally differ from device to device. Thus, this approach was using precious board real estate, and adds cost.
Therefore, there is a need for an alternative system and method of mode stabilization that does not have the drawbacks discussed above relative to conventional solutions.